The post Simple Comma Rules 3-4 is part of a series called Writing Tips. This series is devoted to helping you become a better and more effective writer. To see previous Writing Tips posts, click here.
Two weeks ago, I talked about two simple comma rules: clarifying information in lists and separating clauses in complex sentences. Today we will talk about offsetting additional information and separating independent clauses. I want to note that these are not the only comma rules. Comma rules are plentiful, but they also have a tendency to be confusing. These four rules should cover most of the important rules. At the bottom of this article, you will find a link to a website that has a more comprehensive list of comma rules.
To offset additional information
Rule #3: Use commas to offset additional information or interjections and asides.
Additional information here means that you could remove the information without interfering with the meaning of the sentence. This has a tendency to be a bit subjective, so I will demonstrate with a few examples.
My brother, whose name is Cameron, will visit us in October.
If we take out the information offset by commas–“whose name is Cameron”–we are left with the following sentence: “My brother will visit us in October.” Even without the information about my brother’s name, you can still understand whom I mean. In other words, my brother’s name is not absolutely necessary to the meaning of the sentence. Need another example?
The red house, which I used to live in, is up for sale.
Again, if we remove the information offset by commas–“which I used to live in”–we are left with the sentence: “The red house is up for sale.” This sentence is easy to understand just the way it is.
Hint: Information that should be offset usually begins with something other than that. The word that is usually indicative of necessary information, and thus, does not require a comma.
You can also use commas to offset interjections or asides.
I wanted to go, of course, but I could not.
To separate two independent clauses
Rule #4: Use a comma (+ conjunction) to signal when one independent clauses ends, and another independent clause begins*.
In this case, we must also use coordinate conjunctions (and, or, but, so) to connect the clauses. If you use a comma without a conjunction to connect two independent clauses, you will be committing the dreaded comma splice sin, and no one wants that. You can also use a semi-colon, but I believe that’s another subject for another day.
Now, back to independent clauses. Earlier this month, I talked about using commas to separate the dependent and independent clauses when the dependent clause comes first. In this rule, we’re focusing on independent clauses only. In other words, when we want to combine two sentences that could stand on their own into one single sentence, we use a comma and a coordinate conjunction.
The child wanted to go, but his mother said no.
Good grammar is not something that you can achieve overnight, and even people who love grammar struggle with it on occasion. [Grammar Awareness and You: Part 2]
It is probably easier to catch someone else’s mistakes, but that does not mean you are guaranteed to catch ALL mistakes the first time through. [Editing Myth #3]
*Keep in mind that with short sentences (such as “He left and she cried.”), the comma can be optional. At this point, it’s a stylistic choice, since the lack of comma does not interfere with the meaning of the sentence. Some people may even argue that a comma in the above sentence (so that it read “He left, and she cried.”) would disrupt the flow of the sentence.
For other comma rules, try a site like grammarbook.com.